1. Field of the Invention
This invention relates to compositions for forming flexible articles, sheets or films of glass, glass-ceramic, or ceramic particulate material useful in the manufacture of thin dielectric films or layers.
2. Prior Art
The casting of suspensions of ceramic material to form layers or sheets which are then sintered to produce ceramic substrate material is known in the art. Heretofore, however, the resulting substrate material tended to contain voids which would reduce the insulating or dielectric efficiency of the material. Where the substrate is to be used in an electronic device, such as a capacitor, lack of voids and very high dielectric efficiency are of the utmost importance.
More specifically, where the substrate material is to be utilized in forming what is referred to as a "stacked" capacitor, it is important that the material have good flow characteristics when the stack of capacitor elements is subjected to pressure. In this technique, a plurality of thin green films of, for example, the ceramic material are coated with the necessary conductive patterns and contacts and are arranged in layers. The resulting stack is then subjected to heat and pressure to sinter the green films and form an integral layered article. If the ceramic material does not flow properly under pressure the resulting device will have voids which reduce the insulating nature of the layers and can lead to failure of the device. The development of capacitors having greater capacitance per volume has necessitated the formation of thinner green films than were heretofore required. To achieve such capacitance densities, films as thin as a few mils are necessary. Although films as thin as about one mil can be produced in accordance with the teachings in U.S. Pat. No. 2,966,719 issued to J. L. Park, Jr., the binder utilized in this patent requires the use of organic solvents which are disadvantageous in that they present health and fire hazards and require the use of additional equipment for ventilation purposes.
Some prior art slip compositions can be thinned by the addition of water thereto and therefore are not subject to the aforementioned disadvantages associated with organic solvents. However, the viscosities of those compositions cannot be lowered to the extent necessary for forming thin green films of adequate strength. For example, the slip composition taught in U.S. Pat. No. 3,472,803 issued to P. W. Andrews et al., which includes an aqueous latex emulsion, is useful for the formation of microcircuit substrates having thicknesses greater than about 20 mils. When a sufficient amount of water is added to the Andrews et al. slip composition to form films having thicknesses between about 15 and 20 mils, the resultant green films possess only a minimum of strength. To form films having a thickness up to 15 mils utilizing the Andrews et al. type of binder, such a large amount of water must be added to the slip composition that the resultant green films possess insufficient strength to be handled for subsequent operations.
Another prior art composition for casting ceramic articles is taught in U.S. Pat. No. 2,952,877 issued to J. L. Park, Jr. This patent teaches a water-soluble volatilizable polymeric organic binder such as polyvinyl alcohol. In accordance with the teachings of this latter mentioned Park, Jr. patent, inorganic particulate materials mixed with polymeric organic binder, plasticizer, wetting agent and water to form a heavy paste which is used in the extrusion of relatively thick dielectric bodies such as the 0.040 inch slab specifically mentioned therein. Attempts to thin the Park, Jr. composition by the addition of sufficient water to decrease the viscosity thereof to a value sufficiently low to permit the formation of green films thinner than about 15 mils met with the same problem that was encountered in the attempts to use the Andrews et al. composition. That is, green films having thicknesses up to about 15 mils formed from compositions taught in the Andrews et al. and the latter mentioned Park, Jr. patents possessed insufficient strength when sufficient water was added to the batch to permit the casting of such thin films.
According to the present invention, thin, flexible films of glass, glass-ceramic, or sinterable ceramic particles can be formed which yield excellent dielectric substrate material upon sintering.